Phosphorus based flame retardants can be used for flame retarding engineering plastics like PC/ABS, ABS, HIPS and modified HIPS. Crude phosphate products have been heretofore purified by dissolving the phosphate in non-water miscible solvents such as toluene, heptane, xylenes or dibutyl ether, and then subjecting the non-water miscible solvent/product solution to a series of hot aqueous washes. The typical wash procedure includes acid washes to remove the catalyst.
Examples of these routes are described in European Patent Application Publication No. 690063 and Japanese Patent Publications Nos. 98310593, 09192506 and 08176163. However, any such treatment is followed by multiple water washings to remove slight amounts of acids or alkaline metals salts remaining in the phenylphosphate esters after the phase separation.
WO 98/35970 teaches that liquid arylphosphate esters can be used without any purification to remove the MgCl2 catalyst. However, this approach overlooks corrosion problems originating from Cl− remaining in the product.
The multiple aqueous wash solutions physically separated from the non-water miscible solvent/product solution which are mentioned above, have to be disposed of creating considerable aqueous waste with a relatively high content of organics. It would be desirable to develop a less complex process which reduces the amount of aqueous waste and organics generated in this liquid/liquid extraction process of purifying phosphates without compromising the yield and quality.